The Tucana dwarf spheroidal: a distant backsplash galaxy of M31?
Bibliographic record
Abstract
ABSTRACT We use the APOSTLE Local Group (LG) cosmological hydrosimulations to examine the properties of ‘backsplash’ galaxies, i.e. dwarfs that were within the virial boundaries of the Milky Way (MW) or M31 in the past, but are today outside their virial radius (r200). More than half of all dwarfs between $1\text{ and }2\, r_{200}$ of each primary are backsplash. More distant backsplash systems, i.e. those reaching distances well beyond $2\, r_{200}$, are typically close to apocentre of nearly radial orbits, and, therefore, essentially at rest relative to their primary. We use this result to investigate which LG dwarfs beyond ∼500 kpc of either primary could be a distant backsplash satellite of MW or M31. Tucana dSph, one of the few known quiescent LG field dwarfs, at dM31 ≈ 1350 kpc and dMW ≈ 880 kpc, is a promising candidate. Tucana’s radial velocity is consistent with being at rest relative to M31. Further, Tucana is located close to M33’s orbital plane around M31, and simple orbit integrations indicate that Tucana may have been ejected during an early pericentric passage of M33 ∼11 Gyr ago, a timing that approximately coincides with Tucana’s last episode of star formation. We suggest that Tucana may have been an early-infalling satellite of M31 or M33, providing a compelling explanation for its puzzling lack of gas and ongoing star formation despite its isolated nature. In this scenario, M33 should have completed some orbits around M31, a result that may help to explain the relative dearth of M33 satellite candidates identified so far.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".